First discoveries of z ̃ 6 quasars with the Kilo-Degree Survey and VISTA Kilo-Degree Infrared Galaxy survey

Abstract

We present the results of our first year of quasar search in the ongoing ESO public Kilo-Degree Survey (KiDS) and VISTA Kilo-Degree Infrared Galaxy (VIKING) surveys. These surveys are among the deeper wide-field surveys that can be used to uncover large numbers of z ̃ 6 quasars. This allows us to probe a more common population of z ̃ 6 quasars that is fainter than the well-studied quasars from the main Sloan Digital Sky Survey. From this first set of combined survey catalogues covering ̃250 deg<SUP>2</SUP> we selected point sources down to Z<SUB>AB</SUB> = 22 that had a very red i - Z (i - Z > 2.2) colour. After follow-up imaging and spectroscopy, we discovered four new quasars in the redshift range 5.8 < z < 6.0. The absolute magnitudes at a rest-frame wavelength of 1450 Å are between -26.6 < M<SUB>1450</SUB> < -24.4, confirming that we can find quasars fainter than M*, which at z = 6 has been estimated to be between M* = -25.1 and M* = -27.6. The discovery of four quasars in 250 deg<SUP>2</SUP> of survey data is consistent with predictions based on the z ̃ 6 quasar luminosity function. We discuss various ways to push the candidate selection to fainter magnitudes and we expect to find about 30 new quasars down to an absolute magnitude of M<SUB>1450</SUB> = -24. Studying this homogeneously selected faint quasar population will be important to gain insight into the onset of the co-evolution of the black holes and their stellar hosts.